You can use a Compact Florescent to check.

http://www.vintagetvsets.com/temp/cfl.jpg

Brilliant!

Will that work with DC also? Or just from capacitive coupling with AC? I think I know the answear but wonder what experiance knows.

Pretty sure it's AC only, it works from the Flyback to the middle of the HV rectifier tube but shuts off as soon as you get to the DC side.

It'll sometimes light a little on the 6BG6 output tube too.

I find that if you use a fluorescent tube, ground one end of it, and bring the high voltage anode clip close to the glass in the middle of the tube (not the other metal end of the tube) it will light it. It has to be nearly touching the tube.
Paul